For reproducing acoustic signals, with the use of a pair of headphone units which, on being attached to a listener's head, are supported in the vicinity of fastener's auricles, as in the case of a headphone device adapted for reproducing acoustic signals by a headphone unit, there is so far known a binaural system as a method for optimizing a sound image sense or direction feeling or an external stationary sound image feeling.
With an acoustic reproducing system of the binaural system, acoustic signal reproduced by the headphone device are previously processed in a predetermined manner, as described for example in Japanese Patent Kokoku Publication No. 53-283 (1978).
The sound image sense feeling or the external fixed sound image feeling its determined by sound volume difference, timing difference or phase difference between the sound heard by the left ear and that heard by the right ear.
The above mentioned signal processing is such a signal processing in which, when the acoustic sound is to be reproduced by speaker devices arranged at a distance from the listener, the acoustic effect equivalent to those produced by reflection or diffraction in the vicinity of the listener's head or the difference in the distance between the sound source or the speaker devices and the listener's left and right ears will be produced in an acoustic output reproduced by the headphone devices. Such signal processing is performed by convolutional integration of an impulse response corresponding to the acoustic effects or acoustic signals for left and right ears.
It is noted that, when the acoustic sound is to be reproduced by the speaker devices arranged at a distance from the listener, since the absolute position of the sound image remains unchanged even if a listener should make bodily movements or turn his head, the relative sense and position of a sound image felt by the listener are changed. On the contrary, when the acoustic sound is to be reproduced by the binaural system using the headphone device, since the headphone device is rotated with the listeners head when the listener turns his head, the relative sense and position of the sound image felt by the listener are not changed.
In this manner, in case of binaural reproduction by the headphone device, since a sound field is produced within the listener's head by the difference in the shifting state of sound image relative to changes in the sense of the listener's head, it becomes difficult to fix a sound image ahead of the listener. Besides, the forward sound image tends to be raised in its position.
Heretofore, as described in Japanese Patent, KOKAI Publication No. 42-227 or Japanese Patent Kokoku Publication No. 54-19242, there is proposed an acoustic signal reproducing system in which, by detecting changes in the sense of the listener's head, and changing the state of the signal processing based on the detected results, a satisfactory forward fixed sound image feeling may be obtained in the headphone device with this type of the acoustic signal reproducing system, a sense detection unit, such as a so-called gyrocompass or a magnetic needle, is attached to the listener's head. On the basis of the detected results by said sense detection unit, a delay circuit or the aforementioned level adjustment circuit, processing acoustic signals is controlled to produce a sound field feeling similar to that produced in acoustic reproduction by speaker devices arranged at, a distance from the listener.
Meanwhile, with a conventional binaural reproducing system in which the sense detecting unit such as the gyrocompass is provided in the headphone device, by controlling the contents of the signal processing of the acoustic signals as a function of changes in the sense of the listener's head, a satisfactory fixed sound image feeling may be acquired, as a principle, insofar as the listener remains at a predetermined position.
However, since the sense detecting device for detecting changes in the sense of the listener's head becomes large in size and weight, the construction necessarily needs to be of a stationary type with the fixed listening position.
That is, the sense detection device, such as a gyrocompass, is too large in size and weight to be attached to the listener's freely mobile head, and is not practically usable with a portable type headphone device.
On the other hand, if the listener should make bodily movements, the sound image is also shifted, so that there results an unspontaneous fixed sound image feeling.
When the listener approaches a sound source, such as a speaker device, the round pressure level is usually increased. On the other hand, since the sound source, such as a speaker device, has directivity, the effects of directivity are demonstrated by the listener's bodily movements. This gives rise to an outside fixed sound image feeling.
It is therefore an object of the present invention to provide an acoustic signal reproducing apparatus in which, in view of the above described status of the art, an extremely natural fixed round image feeling without the imaginary sound source position being moved with the headphone device despite the listener's occasional bodily movements may be realized to enable satisfactory binaural reproduction.
It is another object of the present invention to provide an acoustic signal reproducing apparatus in which stable binaural reproduction may be achieved with the headphone device attached to the listener's freely mobile head.
It is a further object of the present invention to provide a headphone device having a head turning angle detecting function whereby changes in the sense of the listener's may be detected quickly, accurately and stably.